A system has been under development in this laboratory to simplify study of human cell-surface antigens by procedures which permit study of antigens due to individual chromosomes. Production of human-Chinese hamster somatic cell hybrids containing specific human chromosomes is achieved by carrying out fusion between selected Chinese hamster auxotrophic mutants and human cells in medium lacking the specific nutrilites required by the Chinese hamster auxotroph. Colonies arise only from hybrid cells containing the particular human chromosome complementing the auxotroph's deficiency. These hybrids which rapidly lose unnecessary human chromosomes can then be tested with antisera produced by immunization of experimental animals with a variety of normal and malignant human cells. Single cell plating procedures have been developed which make it possible to test for the presence of specific human antigens without complications due to the presence of the Chinese hamster cell antigens. Studies of this kind should make possible rapid and definitive identification of a large variety of human cell-surface antigens which may influence malignant behavior of human cells. The chromosomal location and biochemical identity of such genetic determinations will be investigated. These studies may help establish genetic relationships of antigens involved in the malignant response.